Hi guys,
working as an AV service tech, I have found the need for a portable amp to test 100V line systems.
I was thinking along the lines of this circuit by silicon chip

I'm guessing the ouput of the LM386 will be too low to run more than a few speakers tapped at 40W.

Would I be able to use a 20W / 40W bridgemode amplifier (ah-la radioshack) instead?
Can i still use a speaker transformer in reverse, as per the sil-chip circuit?
Can i (long shot) make it battery portable?

Yes, a cheap chip amplifier driving a mains transformer in reverse will do. No need to use a proper audio transformer if it is just for testing. For example, for an amplifier rated at 10W into 8 ohms, that's about 9 volts RMS. Use a 10 watt transformer with a 24V secondary and 230V primary , or 12V secondary and 110V primary.